Algae or no algae?

[Keithj]

I'd appreciate the wisdom of the forum on a strange phenomenon on our house.

The lime render was all done at the same time, three years ago. I've been doing repairs in various places since, but haven't touched the east wall (not needed to).

One area on the east wall grows algae, like this:

whereas an area on an adjacent area (also east wall) doesn't, like this:

Can anyone suggest why one wall is prone to turning green, and the one next to it isn't? Or how I can stop it turning green!

Disregard the tannin on the wall - that's a direct result of the torrential rain we've had these past couple of weeks. I'll be out there to clean it off the lead as soon as it stops raining, and I'll sort the lime areas at the same time.
Likewise, the "wet look wall" is the result of the same (the south bits got drenched).

 

[LadyArowana]

Is that the wall that the neighborhood tom cats pee against?

 

[philpjuk100]

We have algae but only on a west faceing wall,all other walls which have the same finish are algae free.

 

[skier-hughes]

Could it be because of the closeness of the vegetation to the algae wall?

 

[Lime]

Is that the wall that the neighborhood tom cats pee against?

I agree with that diagnosis.

 

[plasticpigeon]

Maybe that wall was done with cement!!! Does look like little widdle marks though!

 

[Penners]

The following suggestion is made with no experience whatsoever of the effect that it might have on lime render/wash (others please add your wisdom).

The usual way of getting rid of algal growth is to wash it with a bleach solution. You might try doing this and seeing whether it regrows. If it does, this could suggest that there is some cause that is continually being renewed (such as animal pee).

 

[Keithj]

Thank you! I'll try bleach and a catapult.

 

[Penners]

Keith - just to reiterate what I said above: I have no experience of the effects of bleach on lime wash or plaster, so I do suggest that you wait until someone more expert than I am either agrees or (more importantly) disagrees with my suggestion. Or, at the very least, you should try a tiny test area first.

 

[DJH]

It can be a result of organic matter in the sand such as 'pussy poo' and is often evidenced in cement render by brown patches that weep a dark stain on the surface.

Doug

 

[MatthewC]

All your algae is roughly vertically aligned, so is it due to water dripping from the cill above? This would have the effect of making the wall damp for a much longer period and hence encourage the algae.

 

[Penners]

That's a smart observation by Matthew. As an additional thought, was the timber above the streaks treated with anything like linseed oil? That could have caused it.

 

[Flyfisher]

Keith - just to reiterate what I said above: I have no experience of the effects of bleach on lime wash or plaster, so I do suggest that you wait until someone more expert than I am either agrees or (more importantly) disagrees with my suggestion. Or, at the very least, you should try a tiny test area first.

It's always a good idea to test a small area first, of course, but this advice suggests it should be OK:

Preparation
The surface to be limewashed should be brushed and washed free of any loose particles, dust, dirt, lichen etc. If there is mould growth the surface should be treated with a fungicide ( e.g. a weak bleach solution) which should be rinsed off before limewashing.
http://www.mikewye.co.uk/mikelimewashing.htm

 

[Keithj]

Thanks for those. No, that wall is normally as dry as the other (non-algae) one. There's nothing above that area that would explain the difference that I can see. I'll try the "clean it off and apply dilute bleach" thing.

There was a different area on another wall that grew algae, but that had a waxy substance on the surface of the lime, which I peeled off and filled with fresh lime putty. That's been fine since.

Maybe a puddytat snuck up and used the lime bucket while the lads were at lunch...

It's a never-ending education, caring for an old house, innit!